OCT is an optical imaging technique which offers non-invasive, cross-sectional imaging of analyzed objects, such as biological tissue. OCT is basically an interferometric technique where the light source is divided into two parts, a reference beam of light propagating in a reference arm and probe beam of light propagating in a sample arm of the interferometric system. The light of the probe beam which is back-scattered from the analyzed object interferes with light of the reference beam reflected from a reflective element in the reference arm and this interference signal produces reflectivity-over-depth information for the analyzed object. Some background information on OCT can, e.g., be found in US 20140078512.
An OCT system generally has a photo-detector, often a balanced one, to record the strength of the interference signal for a range of frequencies, where a characteristic frequency can be mapped directly to an optical path length. Typically, the detector output is converted to a digital signal. These data are then Fourier transformed to reveal reflectivity as a function of optical path length or equivalently depth, which is called an A scan.
Inside absorptive or scattering media, absolute reflectivity becomes small typically at depths of only 1 to 2 mm. At the same time, the reflectivity from the first surface is typically high. This first-surface signal will typically saturate the photo-detector, and hence any frequencies corresponding to higher depths will become undetectable after digitization with a finite bit depth.
There is thus a strong need for OCT systems with greater imaging depths.